1. Field of the Invention
The present invention relates to a method for manufacturing a composite piezoelectric substrate, and particularly, to a method for manufacturing a composite piezoelectric substrate including a piezoelectric film.
2. Description of the Related Art
Recently, filters using ultra-thin piezoelectric materials, such as ultra-thin piezoelectric films, have been actively developed. Although AlN thin films and ZnO thin films formed by a deposition method, such as a sputtering method, a CVD method, or other suitable method are generally used as the ultra-thin piezoelectric films in filters and other similar devices, any one of the films can be a C-axis oriented film in which the C-axis is aligned in the vertical direction of a substrate.
On the other hand, it has been proposed to manufacture a composite piezoelectric substrate including a piezoelectric film by bonding a piezoelectric single crystal substrate to a supporting substrate and then thinning the piezoelectric single crystal substrate by polishing.
For example, a surface acoustic wave device 100 shown in a sectional view of FIG. 6 includes a piezoelectric substrate 101 made of a single crystal and having an excitation electrode 105 provided thereon, the piezoelectric substrate 101 being thinned by polishing in a state in which the piezoelectric substrate 101 is previously bonded to a protective substrate 103 through a glass layer 108 (see, for example, Japanese Unexamined Patent Application Publication No. 2002-16468 and Japanese Unexamined Patent Application Publication No. 2002-16468).
In addition, a method has been proposed, in which hydrogen ions are implanted into a piezoelectric substrate, the piezoelectric substrate and a supporting substrate are disposed in a wet atmosphere to form hydrophilic groups on a surface of each of the substrates and then the substrates are bonded together. Subsequently, the piezoelectric substrate is broken at the depth of the ion implantation by high-speed heating to form a piezoelectric thin film (see, for example, Japanese Unexamined Patent Application Publication No. 2002-534886).
However, a piezoelectric film formed by the deposition method has severe limitations on the materials that can be used due to the deposition temperature and deposition conditions for forming an oriented film, and AlN is primarily used. Also, the orientation direction of a crystal axis cannot be accurately controlled, and a C-axis oriented film is primarily used, thereby causing difficulty in designing a vibration mode by inclining a piezoelectric axis.
On the other hand, a piezoelectric film that is formed by polishing a piezoelectric single crystal substrate results in most of the piezoelectric single crystal becoming polishing waste, thereby causing a low efficiency of material utilization. Further, the thickness of the piezoelectric film depends on variations in the polishing speed and waviness of the substrate before polishing, and thus, it is difficult to accurately control the thickness so as to produce uniform thickness, thereby causing low productivity.
Japanese Unexamined Patent Application Publication No. 2002-534886 uses bonding with hydrophilic groups. This bonding method forms hydrophilic groups on surfaces of the piezoelectric substrate and the supporting substrate so that the substrates are bonded through the hydrophilic groups. Since bonding through the hydrophilic groups is very weak, to achieve strong bonding, it is necessary to perform a step of strengthening the bond between the surfaces of the piezoelectric substrate and the supporting substrate by heating the substrates at a temperature (e.g., about 400° C.) at which the hydrophilic groups can be decomposed to eliminate hydrogen (H) from the hydrophilic groups.
However, there are problems in that gas stays in the bonding interface due to the elimination of hydrogen gas after bonding, thereby forming microcavities, and in that hydrogen gas cannot be sufficiently removed and remains in the piezoelectric crystal, thereby breaking the crystal structure and degrading the piezoelectricity. The non-uniformity at the bonding interface due to the microcavities or the crystal breakage causes generation of heat due to elastic scattering or sound absorption when used in surface acoustic wave filters and bulk acoustic wave filters. This causes a deterioration of insertion loss of a filter and a deterioration of electric power resistance due to heat generation.
In particular, when LiTaO3 or LiNbO3 is used, remaining hydrogen causes substitution of Li at a Li site with hydrogen and easily degrades the piezoelectricity.
For example, a lithium tantalate or lithium niobate substrate is used as a piezoelectric substrate, and a lithium tantalate or lithium niobate substrate is also used as a supporting substrate. The surfaces of the piezoelectric substrate and the supporting substrate are smoothed by CMP (chemical mechanical polishing), and then the piezoelectric substrate and the supporting substrate are exposed to a wet atmosphere to form hydrophilic groups. The surfaces of the piezoelectric substrate and the supporting substrate arranged in contact and bonded together to cause weak bonding with hydrophilic groups. Then, the substrates are heated at 500° C. for hour to cause strong bonding by the decomposition of hydrophilic groups. A composite piezoelectric substrate formed by this bonding method easily produces separation at the bonding interface when an external force is applied before heating at 500° C. On the other hand, bonding is strengthened after heating at 500° C., but visible cavities are formed throughout the bonding interface. In addition, the crystalline arrangement of the bonding interface when viewed using a TEM (transmission electron microscope) shows cavities of several tens of nm and disorder of the crystal arrangement.
Further, when H ions are implanted and a polarization state of LiTaO3 after heating 400° C. is examined with a nonlinear dielectric microscope, it is discovered that polarization polarities that are initially arrayed are locally inverted in domain units, which degrades the piezoelectricity. The piezoelectricity is significantly degraded by heating at a temperature of 500° C.